News Release Number: STScI-2009-11

Hubble Provides New Evidence for Dark Matter Around Small Galaxies

March 12, 2009: When it comes to finding dark matter in space, astronomers need to go on sort of a ghost
hunt. Dark matter can't be directly seen or isolated in a laboratory. Yet it makes up the
bulk of the matter in the universe. It is the invisible scaffolding for the formation of stars
and galaxies. Dark matter is not made of the same stuff that stars, planets, and people are
made of. That stuff is normal "baryonic" matter, consisting of electrons, protons, and
neutrons. For 80 years astronomers have known about dark matter's "ghostly" pull on
normal matter. They've known that without the gravitational "glue" of dark matter galaxy
clusters would fly apart, and even galaxies would have a hard time holding together.

Now the Hubble Space Telescope has uncovered a strong new line of evidence that
galaxies are embedded in halos of dark matter. Peering into the tumultuous heart of the
nearby Perseus galaxy cluster, Hubble's sharp view resolved a large population of small
galaxies that have remained intact while larger galaxies around them are being ripped
apart by the gravitational tug of other galaxies. The dwarfs' "invisible shield" is a robust
halo of dark matter that keeps them intact despite a several-billion-year-long bumper-car
game inside the massive galaxy cluster.